1. Field of the Invention
The present invention relates generally to radiation-sensitive positive photoresist compositions and particularly to compositions containing cresol-formaldehyde novolak resins together with naphthoquinone diazide sensitizing agents.
2. Description of the Prior Art
Positive photoresist formulations such as are described in, for example, U.S. Pat. Nos. 3,666,473, 4,115,128 and 4,173,470, include alkali-soluble phenol-formaldehyde novolak resins together with light-sensitive materials, usually a substituted naphthoquinone diazide compound. The resins and sensitizers are dissolved in an organic solvent or mixture of solvents and are applied as a thin film or coating to a substrate suitable for the particular application desired.
The novolak resin component of these photoresist formulations is soluble in alkaline aqueous solution, but the naphthoquinone sensitizer acts as a dissolution rate inhibitor with respect to the resin. Upon exposure of selected areas of the coated substrate to actinic radiation, however, the sensitizer undergoes a radiation-induced structural transformation which decreases its efficiency as a dissolution rate inhibitor for the novolak and, subsequently, the exposed areas of the coating are rendered more soluble than the unexposed areas. This difference in solubility rates causes the exposed areas of the photoresist coating to be dissolved when the substrate is immersed in alkaline developing solution while the unexposed areas are largely unaffected, thus producing a positive relief pattern of photoresist on substrate.
The naphthoquinone diazide sensitizer compounds typically used in positive photoresists are highly absorptive when exposed to radiation in the conventional ultraviolet to near-ultraviolet range, i.e., radiation with a wavelength of from 400 to 450 nanometers, and the sensitizers are therefore quite photoactive in that range. Moreover, the sensitizers generally bleach in the conventional or near-ultraviolet range, i.e., the photoproducts of the radiation-induced structural transformation caused by the exposure of the sensitizer to the radiation are non-absorptive in that range of wavelengths and therefore the radiation is able to penetrate to the next layer of sensitizer after the exposure of the uppermost layer. The novolak resin itself absorbs very little in the 400 or 450 nanometer range and therefore the conventional resists prepared from the novolak resins and naphthoquinone diazide sensitizers are essentially transparent to radiation with these wavelengths and the radiation is able to penetrate the resist coating down to the interface between the coating and the substrate.
In most instances, the exposed and developed substrate will be subjected to treatment by a substrate-etchant solution. The photoresist coating protects the coated areas of the substrate from the etchant and thus the etchant is only able to etch the uncoated areas of the substrate, which, in the case of a positive photoresist, correspond to the areas that were exposed to actinic radiation. Thus, an etched pattern can be created on the substrate which corresponds to the pattern of the mask, stencil, template, etc., that was used to create selective exposure patterns on the coated substrate prior to development.
The relief pattern of photoresist on substrate produced by the method described above is useful for various applications including, for example, as an exposure mask or a pattern such as is employed in the manufacture of miniaturized integrated electronic components.
The properties of a photoresist composition which are important in commercial practice and which are sought to be improved hereby include the photospeed of the resist, the development contrast thereof, the resist resolution, and the resist adhesion.
For the purposes of this specification, resist photospeed is defined as the minimum exposure time, assuming constant exposure energy per unit of time, required to render an exposed area of a dried resist coating of a given thickness on a substrate completely soluble to a developing solution. Increased photospeed is important for a photoresist, particularly in applications where a number of exposures are needed, for example, in generating multiple patterns by a repeated process, or where light of reduced intensity is employed such as, for example, in projection exposure techniques where the light is passed through a series of lenses and monochromatic filters. Thus, increased photospeed is particularly important for a resist composition employed in processes where a number of multiple exposures must be made to produce a mask or series of circuit patterns on a substrate. In measuring resist photospeed, optimum development conditions are utilized. These optimum conditions include a constant development temperature and time in a particular development mode, and a developer system selected to provide complete development of exposed resist areas while maintaining a maximum unexposed resist film thickness loss not exceeding 10 percent of initial thickness.
Development contrast, as used herein, refers to a comparison between the percentage of film loss in the exposed area of development with the percentage of film loss on the unexposed area. Ordinarily, development of an exposed resist coated substrate is continued until the coating on the exposed area is completely dissolved away and thus, development contrast can be determined simply by measuring the percentage of the film coating loss in the unexposed areas when the exposed coating areas are removed entirely. By definition, development contrast is always within an acceptable range under optimum development conditions because these conditions are selected to provide no more than 10 percent loss of film thickness in the unexposed areas, while the coating or film in the exposed areas is completely removed.
Resist resolution is defined as the capability of a resist system to reproduce the smallest equally spaced line pairs and intervening spaces of a mask which is utilized during exposure with a high degree of image edge acuity in the developed exposed spaces. Quantitatively, resist resolution can be measured by calculating, after development, the ratio of the width of the unexposed area corresponding to a line on the mask to the width of the exposed area corresponding to the adjacent space and comparing that ratio to the ratio of line and space widths on the mask itself. Ideally, the ratio of the widths on the developed resist-coated substrate should equal the ratio of the widths on the mask. In many industrial applications, particularly in the manufacture of miniaturized electronic components, a photoresist is required to provide a high degree of resolution for very small line and space widths (on the order of one micron or less).
It has been well established in the photoresist art that utilizing radiation in the deep ultraviolet range, for example, with a wavelength of 250 or 270 nanometers, for exposure of resist-coated substrates results in a great increase in resolution in the developed resist and enables the reproduction of lines and spaces of very small dimensions with high image-edge acuity. The ability of a resist to reproduce very small dimensions, on the order of a micron or less, is extremely important in the production of large scale integrated circuits on silicon chips and similar components. Circuit density on such a chip can only be increased, assuming photolithography techniques are utilized, by increasing the resolution capabilities of the resist. Although negative photoresists, wherein the exposed areas of resist coating become insoluble and the unexposed areas are dissolved away by the developer, have been extensively used for this purpose by the semiconductor industry, positive photoresists have inherently higher resolution and are beginning to be utilized as replacements for the negative resists.
There are two principal problems with the use of conventional positive photoresists in the production of miniaturized integrated circuit components. One is that the positive resists have slower photospeed than the negative resists. The second drawback with conventional positive photoresists is that the naphthoquinone diazide sensitizers which generally comprise the photosensitive component of these resists absorb substantially in the deep ultraviolet range where highest resolution can be achieved and the sensitizers must be used in substantial concentrations to be effective as solubility inhibitors, and thus the resists are somewhat opaque to radiation within this range and cannot be developed down to the interface with the substrate because the radiation cannot penetrate through the upper layers of the resist coating.
Various attempts have been made in the prior art to improve the photospeed of positive photoresist compositions. For example, in U.S. Pat. No. 3,666,473, a mixture of two phenol-formaldehyde novolak resins was utilized together with a typical sensitizer, said novolak resins being defined by their solubility rates in alkaline solutions of a particular pH and by their cloud points. In U.S. Pat. No. 4,115,128, a third component consisting of an organic acid cyclic anhydride was added to the phenolic resin and naphthoquinone diazide sensitizer to provide increased photospeed. These prior art compositions, however, while providing somewhat improved photospeed, involve the use of additional components or overly complex control of reaction conditions and detailed analysis of the components utilized, in comparison with compositions utilizing simply a single phenolic resin together with a simple sensitizer.
Another prior art photoresist composition, which has been commercially available, contains a novolak resin formed from a mixture of ortho-, meta-, and paracresols and subsequently chemically reacted with a particular naphthoquinone diazide sensitizer to produce a sensitized novolak; e.g., a novolak chemically bonded to a sensitizer. This prior art material, while possessing good resolution properties, does not have particularly rapid photospeed, which is a significant drawback in many industrial applications such as the large scale production of miniaturized integrated circuit components.
Moreover, most of the prior art positive photoresists which exhibit increased photospeed use substantial concentrations of sensitizers which absorb and do not bleach well in the deep ultraviolet radiation range and thus these resists cannot effectively be used to produce the very high resolution small line-and-space widths required in the production of microcircuitry components. Those resist compositions which have been developed using sensitizers that do bleach in the deep UV are generally not usable in the more conventional exposure ranges because they absorb so little in the 330 to 450 nanometer range, and are thus not rapidly transformed on exposure, that they exhibit very poor photospeed in these radiation ranges.
No prior art positive photoresist possesses high photospeed and has good resolution properties when exposed to deep UV radiation and yet is useful in the conventional exposure ranges as well.